A low-construction trolley for a wire rope hoist, arranged to travel along a lower flange of a horizontal beam or rail, whereby the trolley comprises a trolley frame; bearing wheels that are attached to the frame of the trolley and arranged to travel on the upper surface of the lower flange of the beam or rail, and at least some of which are driven wheels to move the trolley; a hoisting mechanism comprising a rope drum for a hoisting rope, a hoisting member in cooperation with the hoisting rope for hoisting a load, and a hoisting motor for driving the rope drum; whereby the rope drum is supported to a first side of the trolley frame so that the axle of the rope drum is parallel to the beam or rail, and the hoisting member is arranged to move under the beam or rail; whereby the hoisting rope is led from the rope drum to a fixing point in the trolley frame through at least a rope pulley arrangement of the hoisting member.
A low-construction trolley is a common, overhead trolley type for lightweight bridge cranes with one main support. The basic idea behind it is to save hall height by taking up as little space as possible in the vertical direction.
As a result of the external shape taking up little space, the heaviest parts of a trolley in prior art, that is, the hoisting mechanism and rope drum, are found on the same side of the main support (which is the same as the aforementioned beam of rail or comprises the aforementioned beam or rail). This causes imbalance on the opposite sides of the main support, which is typically compensated for by a counterweight on the opposite side of the main support and opposite to the hoisting mechanism. When examining the hoist in the vertical direction, the rope system and hoisting member of the hoisting rope are in such a case arranged in an area limited by the ends of the counterweight, hoisting mechanism, and rope drum, whereby the fastenings of the hoisting rope to the rope drum and the trolley frame are also located in this area. Due to this space problem, the fixing point of the hoisting rope to the trolley frame is found closer to the main support than the disengagement point of the hoisting rope from the drum. For example, in a 4-rope solution, the fixing point of a sheave, in such a case, is to be placed at the mean distance of the aforementioned fixing point and disengagement point from the main support on the opposite side of the main support for the hoisting member to move essentially in the vertical plane that runs through the main support and for the trolley to stay in balance. This asymmetry and long lever arms in relation to the main support cause, however, additional twisting of the rope system in relation to the vertical axis in a load hoisting and lowering situation, and consequently harmfully large bending moment and the resulting additional stress on the trolley, on the bearing wheels, for example, and thus additional stress also on the main support. Disproportional load may cause premature wear and tear on the bearing wheels and/or the guiding surfaces on the beam or rail that the main support potentially has, creating jerking movement for the trolley.
In addition, if the fixing of the hoisting rope to the rope drum is typically located on the side of the trolley where the fixing point of the hoisting rope to the trolley frame is also located, the portion of the rope system that leads to a fixed fixing point may hit the portion of the rope system coming from the rope drum when the hoisting member is being lowered, which may wear and damage the hoisting rope. In this solution, the rope base, that is, the projection of the rope system in the horizontal plane is reduced as the hoisting member is lowered, which at the same time decreases the geometric resistance of the hoisting member twisting.
The “asymmetric” rope system described above is also implemented in such trolleys where the counterweight is replaced with a suspended contact to the main support, that is, the lower surface of the lower flange of the horizontal beam or rail referred to in the above. The U.S. Pat. No. 7,234,400 B2 and EP 0 620 179 B1 serve as examples of this solution. In these, too, exactly the same problems are seen as in trolleys comprising a counterweight.